We have measured the surface roughness profile using a stylus instrument
[Mitutoyo Portable Surface Roughness Measurement Surftest SJ-410 with a
diamond tip with the radius of curvature \(r_0 =1 \ {\rm \mu m}\) , and with the tip-substrate repulsive force \(F_{\rm N} = 0.75 \ {\rm mN}\) 
and the tip speed \(v=50 \ {\rm \mu m/s}\) ],
and calculated the surface roughness
power spectrum as described in detail elsewhere[1]}.
Fig. REF  shows the 1D surface roughness power spectra of the sandblasted O-rings O1-O5,
and for a not sandblasted O-ring.
Fig. REF 
shows the 2D surface roughness power spectra obtained from 1D line scans from the sandblasted rubber O-rings
O3a, O6a, O8a, O9a, O10a.
In each case the power spectra was obtained by averaging over 7 line scans, each \(2 \ {\rm mm}\) 
long in the direction of the largest O-ring curvature radius.
